In the context of the present invention, spectacle lenses are understood to mean all types of optical corrective lenses and also lenses without optical correction which are part of spectacles and through which the wearer of spectacles looks during use as intended. There are spectacle lenses composed of plastic and those composed of mineral glass.
Polarization describes the direction of vibration of an electromagnetic wave. Natural light is usually perceived in an unpolarized manner as a superposition of different waves of electromagnetic radiation having different vibration planes and phases. After reflection at an interface, the reflected light partly has a polarization direction.
A polarizing spectacle lens is defined in DIN EN ISO 13666 (1998) as a spectacle lens having different light absorption depending on the polarization of the impinging light. The orientation of the maximum transmission of the electric field vector of the electromagnetic radiation through a polarizing spectacle lens is designated as the polarization axis of the spectacle lens. The position of the polarization axis in the frame is specified in degrees (°), wherein 0° describes a horizontal orientation and 90° a vertical orientation of the polarization axis. The transmission plane of a polarizing spectacle lens is defined as a plane which intersects the spectacle lens and contains the direction of propagation of the transmitted radiation and is parallel to the orientation of the maximum transmission of the electric field vector of the transmitted radiation. The polarization plane of a polarizing spectacle lens is normal to the transmission plane and is often identified by markings on a polarizing spectacle lens. A set-up for determining the polarization plane is shown for example in DIN EN ISO 8980-3:2004 or DIN EN 1836:2005+A1:2007 (D).
There are spectacle lenses which permanently have a preferably predetermined polarizing property, and those in which the polarizing property can change. The latter also include so-called phototropic spectacle lenses. Part of such spectacle lenses is a phototropic material, for example, in the form of a coating or in the form of additives to the spectacle lens body. A phototropic material is a material which changes its light transmission properties reversibly depending on the irradiance and the wavelengths of the impinging radiation. In this case, the alteration of the light transmission properties can purely alter the absorption or alternatively produce a polarizing effect.
In the case of polarizing spectacles, the two polarizing spectacle lenses are fixedly incorporated in a frame. This should be understood to mean that there is a mechanically fixed connection between the frame and the spectacle lenses. Therefore, full rim frames, semi-rimless frames, rim frames and also rimless frames can be provided for fixing the spectacle lenses.
Polarizing spectacle lenses are principally used in sunglasses. In the case of such polarizing spectacle lenses, which are intended to reduce glare from the sun, the transmission plane is normally oriented vertically, and the polarization plane horizontally, for the reasons given below.
As is known, the so-called Brewster angle is the angle with respect to the normal to an interface at which incident light is reflected such that only the portions polarized parallel to the interface (that is, perpendicularly to the plane of incidence) are reflected (s-polarized). In the case of a horizontal interface (such as a water surface, for example), light reflected at this angle is therefore horizontally polarized. At other angles deviating from the Brewster angle, the reflected light additionally has polarized portions lying in the plane of incidence (p-polarized). In the case of polarizing spectacles such as sunglasses, for example, comprising polarizing spectacle lenses, the polarization axis is oriented vertically (90°) and the polarization plane is oriented horizontally (0°). Spectacle lenses having a vertical polarization axis or horizontal polarization plane are therefore transmissive to vertically polarized light. Reflections on horizontal surfaces (such as water surfaces, for example) are thus greatly reduced for the wearer of spectacles.
Polarizing spectacles consist of two polarizing spectacle lenses fixedly incorporated in a frame, wherein the two defined polarization planes of the two polarizing spectacle lenses according to DIN EN 1836:2005+A1:2007 should not deviate from one another by more than 6°.
DIN EN 1836:2005+A1:2007 (D) furthermore provides that the polarization plane in the case of sunglasses must not deviate from the horizontal by more than +/−5° . That means that polarizing spectacle lenses have to be incorporated in the frame in such a way that the polarization axis thereof does not deviate from the vertical by more than +/−5° . Such sunglasses are described for example in GB747 235.
The degree of polarization or the polarization efficiency quantifies the quality of the polarization of a spectacle lens. The terms degree of polarization and polarization efficiency are often regarded as equivalent in the literature. The degree of polarization and the polarization efficiency are defined in DIN EN 1836 and DIN EN ISO 13666. The degree of polarization P is defined as P=(Imax−Imin)/(Imax+Imin), wherein Imaxand Imindenote the extremes of the light transmittance I. In order to determine the degree of polarization, the polarizing spectacle lens is irradiated on one side with 100% linearly polarized light having a defined polarization plane and having an intensity. On the opposite side of the polarizing spectacle lens, it is possible to measure the intensity depending on the angular position of the polarizing spectacle lens relative to the defined polarization plane. For a specific angle, the maximum I max of the light transmittance I will attain a maximum. This angle is designated by the polarization axis. At this angular position, every plane parallel to the defined polarization plane of the incident light is a transmission plane of the polarizing spectacle lens. At a different angle, the minimum value Imin of the light transmittance I occurs. Imin usually occurs at an angle which deviates by 90° with respect to the polarization axis. At this angular position, every plane parallel to the defined polarization plane of the incident light is a polarization plane of the polarizing spectacle lens. The indication of the polarization ratio R=Imax/Imin can also be used for assessing the quality of polarizing spectacle lenses.
Polarizing spectacle lenses pursuant to DIN EN 1836:2005+A1:2007 (D) and DIN EN ISO 8980-3:2004 must have a ratio between maximum and minimum light transmission of greater than 8:1 and greater than 4:1, respectively. Polarizing spectacle lenses thus achieve a degree of polarization of 78% and 60%, respectively. High-quality lenses have a degree of polarization of more than 99%.
However, such characteristic figures are variables which a wearer of spectacles can comprehend only with difficulty. Without their own comparison values or values from experience or without knowledge of the significance of the characteristic figures, in general it is virtually impossible for the wearer of spectacles to differentiate high-quality spectacle lenses from lower-quality spectacle lenses.
Therefore, in order to demonstrate the polarizing property of, for example, sunglasses comprising polarizing spectacle lenses, nowadays use is often made of stickers or small emblems which reflect light with a defined polarization direction. In this case, the wearer of spectacles can view the stickers or emblems through polarizing spectacle lenses. If the stickers or emblems are rotated relative to the polarizing spectacle lenses in a frame, then a change in intensity is apparent which does not occur in the case of non-polarizing spectacle lenses. By this means, however, no statement whatsoever about the quality of polarizing spectacle lenses in a frame can be made, but rather only a simple differentiation between polarizing spectacle lenses in a frame and non-polarizing spectacle lenses in a frame.
Further demonstrators are based on a simulated reflection being superposed on a test image by means of semi-transmissive film. When the demonstrator is viewed through polarizing spectacles, the intensity of the simulated reflection is greatly reduced and the original image can be seen with increased contrast. No differentiation of quality is possible in this case either. Moreover, demonstrators of this type exhibit only a single test image, which possibly does not correspond to the use conditions of the wearer of spectacles.
The prior art furthermore discloses vision testing instruments which display optotypes with polarized light having different polarization directions with the aid of optotype charts or displays. In use as intended, a subject wears spectacles comprising two differently polarized spectacle lenses, wherein the polarization axes of the two spectacle lenses are perpendicular to one another. What is achieved by this means is that the subject can perceive a displayed optotype or parts thereof only with one eye and other parts only with the other eye.
A vision testing instrument of this type is known from U.S. Pat. No. 5,331,358, for example. The vision testing instrument substantially consists of two polarization films and two liquid crystal displays, which are arranged alternately. This arrangement is able to influence the light coming from a light source such that optotypes or parts thereof can purposefully be seen only by one eye or else by both eyes. The light from the light source firstly illuminates the first, color-neutral, transparent polarization film. The light passing through the first polarization film penetrates through the first, if appropriate colored, display and illuminates the second, color-neutral polarization film. The light passing through the second polarization film penetrates through the second display, after which the light leaves the vision testing instrument. A vision testing instrument of this type can then be used to represent figures and drawings by virtue of the individual liquid crystal cells being driven. If the optotype represented is intended to be altered, then a corresponding command is given to a switching device via a keyboard, which switching device drives the two driver circuits of the two displays in a desired manner.
U.S. Pat. No. 5,638,082 describes a vision testing instrument substantially consisting of at least one vectograph film with many individual polarization elements and at least one screen. The polarization elements are arranged in strip-shape fashion, wherein the polarization elements of a strip have the same polarization direction. The polarization elements in the strips arranged alongside one another have polarization axes perpendicular to one another. It is explicitly noted in the publication that the liquid crystal displays normally consist of several hundred by several hundred LCD elements, such that no strips are visible when the observer is at a sufficiently large distance from the display.
DE 199 47 775 A1 describes a test device similar to the type described in U.S. Pat. No. 5,638,082. Horizontal pairs of lines arranged one above another are presented to the subject's eyes, wherein the test device respectively provides only one line for one eye and the other line for the other eye.
DE 100 07 020 A1 relates to a near vision testing instrument for displaying optotype charts. One or more optotypes for testing the visual faculty of the user is or are arranged on each optotype chart. An optotype chart shows a stereo test that checks the subject's spatial perception. On account of the differently polarized triangles offset horizontally, the latter appear spatially offset with respect to a point which is arranged in the center of the optotype chart, is represented black and is perceptible to both eyes.
These vision testing instruments are configured for spectacles comprising polarizing spectacle lenses whose polarization axes form an angle that differs from zero, usually a 90° angle. Although an optotype currently displayed by the display of the respective vision testing instrument can bring about a different visual impression for the wearer of spectacles comprising polarized spectacle lenses having polarization axes oriented parallel, depending on the quality of the polarization properties, the wearer of spectacles generally cannot assess on the basis of the visual impression whether they are wearing high-quality or low-quality spectacles.
United States patent application publication 2006/0203338 A1, from which the invention proceeds, describes a stacked display for representing three-dimensional images and videos. Two superposed, polarized image patterns are generated, which differ by virtue of different polarization planes. The two image patterns bring about a three-dimensional visual impression for the observer as a result of simultaneous viewing through spectacles comprising polarized spectacle lenses having polarization axes that differ by 90°. A three-dimensional visual impression can be generated only if the two image patterns show an object from different perspectives. The two image patterns are therefore not congruent. The motifs of the two image patterns are therefore not identical. If the display is viewed using spectacles comprising polarized spectacle lenses having polarization axes oriented parallel to one another, either only one of the two image patterns is perceived by one of the two eyes, specifically if the polarization direction of the light from this image pattern precisely coincides with the orientation of the polarization axis of the spectacle lenses, or both image patterns are perceived simultaneously by both eyes, which brings about a blurred visual impression because the two motifs are not identical and not congruent. A statement about the quality of the polarized spectacles is not possible.